cold bridge and dew point in action.
Not thats its any help to you, but its why new build regs windows have to be sat in the cavity at least 70% so they form part of the insulation envelope of the building and then issues like yours are moot.
i've had every single isolator wired back to the plant room, not that there is many of them. Nothing worse than seeing them splattered all over a build.
There really isnt much to an ASHP, a flow and return, 2 electric feeds and thats it.
Much simpler and quicker to fit than a gas combi for example.
You could DIY the lot for £5k tops, obvs get a sparky in for the hook up.
Heads up another octopus session today mid day, paid to use as much as you can.
16kw is the cap with everything on for us unfortunately. Was hoping for 20.
our setup similar to johnmo, albeit we control all automation with home assistant. Heat pump is always on automatic 24x7 either for cooling or heating. hot water replenishes to 40 via ASHP immediately, then depending on the time of day (solar availability) it checks the 30min octopus price for cheapness and if its cheap it blasts to 55 after which the eddi (ASHP has no direct control of the immersion) takes it to 65.
Took a bit of tuning to get setup but now its just set and forget.